A carrier protein or complex of proteins that transports protons (H⁺) across biological membranes. Such pumps use energy, for example ATP, to establish a high concentration of protons on one side of the membrane compared to the other. This proton gradient is then exploited by the cell to drive various processes, including the transport of ions and small molecules across plasma membranes. Proton pumps are also involved in secretion of gastric acid by mucosal cells in the stomach lining, the uptake of mineral ions by plant root hairs, and the expansion of plant cell walls induced by auxins. The concept of a proton pump is central to the chemiosmotic mechanism of ATP formation by an electron transport chain in mitochondria and chloroplasts. Here, the pump is driven not by ATP hydrolysis but by electron transport, and the proton gradient that is set up represents a means of conserving the energy released by respiration, or captured by photosynthetic pigments. This energy is then stored as ATP by the enzyme ATP synthetase. See also acid growth theory.